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undergoes during the formation of the tadpole. 
this respect corresponding exactly with what happens to the 
pullet's egg. ■ The first change that is produced towards the 
formation of an embryo is, the contents of the vesicle ex- 
pand, its form changes from that of a sphere to an oval, and 
when cut through its contents are no longer fluid. In the 
act of coagulation, the central portion becomes of a lighter 
colour than that which surrounds it, swells out in the middle, 
and there is a distinct line by which the two portions are 
separated from one another : the central part, in its future 
changes, is converted into brain and spinal marrow, and after < 
these organs have acquired a defined outline, the heart and 
other viscera are seen forming in the darker substance. 
This does not exactly correspond with what takes place 
in the pullet's egg, that of the frog having no yelk. In the 
pullet's egg, the part within the inner circle of the molecule, 
when impregnated by the male, undergoes the necessary 
changes to form the brain and spinal marrow ; the part 
within the outer circle forms the blood and its vessels ; the 
supplies out of which the other organs are to be produced, 
are afterwards derived from the yelk. 
The membrane that forms the vesicle which is destined to 
contain the embryo when it has become a tadpole, has a 
power of enlargement as the embryo increases in size, and 
then performs the office both of the shell and of the mem- 
brane that lines it in the pullet's egg, at the same time serv- 
ing as a defence to protect it, and allow of the blood being 
aerated. 
The nigrum pigmentum lining the vescicle can only an- 
swer some secondary purpose, since it is not met with in the 
aquatic salamander, whose mode of breeding very closely 
